Frequency tuning circuits and voltage-controlled oscillator including the same

ABSTRACT

A frequency fine tuning circuit for use in a voltage-controlled oscillator is provided. The frequency tuning circuit includes a first varactor, a second varactor and a center bias unit. The second varactor is coupled to the first varactor at a first node. The center bias unit maintains a node voltage of the first node at a constant bias voltage level.

CLAIM FOR PRIORITY

This application claims priority under 35 USC § 119 to Korean PatentApplication No. 2005-37581, filed on May 4, 2005, the disclosure ofwhich is hereby incorporated by reference herein in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to frequency tuning circuits and avoltage-controlled oscillator, and more particularly to a fine tuningcircuit, a coarse tuning circuit and a voltage-controlled oscillatorincluding the same that may operate efficiently and stably.

2. Description of the Related Art

Due to the widespread use of mobile communication, mobile communicationterminals capable of offering better quality service are in demand. Forinstance, a voltage-controlled oscillator (VCO) is one of the devicesthat is used for mobile communication terminals, such as e.g., for awide-band receiver. Also, the performance of the voltage-controlledoscillator significantly affects the quality of the mobilecommunication.

It is preferable that the voltage-controlled oscillator have a widefrequency tuning range and a low noise transmission. Suchcharacteristics become more significant when the voltage-controlledoscillator is utilized at the front end of a mobile communicationterminal, where noise may be transmitted through e.g., an antenna, or adigital cable.

A voltage-controlled oscillator includes an active circuit, an LC tankfor frequency oscillation and a tuning circuit for tuning an oscillationfrequency. The tuning circuit may be divided into a coarse tuningcircuit and a fine tuning circuit.

Moreover, the tuning circuit may be implemented in a variety of forms.For example, a conventional tuning circuit typically includes a diodehaving a variable capacitance, wherein the diode has a structure of ap+/n− well junction. However, the above conventional tuning circuit maynot achieve a higher Q value due to the use of the diode, and thus theperformance of the tuning circuit may be lowered.

To overcome the above difficulty, an accumulation MOS (AMOS) varactorhas been adopted for use in a tuning circuit.

FIG. 1 is a cross-sectional view illustrating a conventional structureof an AMOS varactor, which is also described in U.S. Pat. No. 6,211,745.

Referring to FIG. 1, the AMOS varactor may have a P-type gate formed onan N-type well (hereinafter, referred to as a P-Gate/N-well structure)or an N-type gate formed on a P-type well (hereinafter, referred to asan N-gate/P-well structure).

When the AMOS varactor has the P-gate/N-well structure, a source region24 and a drain region 22 of the varactor are N⁺ types regions and well20 of the varactor is an N⁻ type well. Additionally, gate poly 30 of theP-type gate is of a P⁺ type conductivity and contacts 26 and 28 arecomposed of metal. A P-gate terminal 34 is connected to gate poly 30,and contact terminals 32 and 36 are connected to contacts 28 and 26,respectively. Moreover, when the AMOS varactor has the N-gate/P-wellstructure, the source region 24 and the drain region 22 are P⁺ typesregions and the well 20 is a P⁻ type well. Furthermore, gate poly 30 hasan N⁺ type conductivity and contacts 26 and 28 are composed of metal.

FIG. 2 is a graph illustrating a tuning characteristic of theconventional AMOS varactor in FIG. 1. FIG. 2 shows that the capacitanceCmos of the AMOS varactor is varied according to a tuning voltage Vtuneranging from −2V to +2V,

As shown in FIG. 2, the capacitance Cmos of the AMOS varactor exhibits asignificant change with respect to the tuning voltage Vtune ranging fromabout −1V to about 1V. In contrast, the capacitance Cmos of the AMOSvaractor exhibits little change with respect to the tuning voltage Vtuneoutside of the range of about −1V to about 1V.

FIG. 3 is a circuit diagram illustrating a conventional fine tuningcircuit. The conventional fine tuning circuit in FIG. 3 is alsodescribed in U.K. Patent Application No. GB2379104, Japan PatentLaid-Open Publication No. 2003-229718.

Referring to FIG. 3, the conventional fine tuning circuit includes afirst varactor Cv1 and a second varactor Cv2 that are serially coupledto each other. The first varactor Cv1 and the second varactor Cv2 mayinclude an AMOS varactor.

The first and second varactors Cv1 and Cv2 have gate terminals coupledto a first output terminal 11 and a second output terminal 12,respectively and source/drain terminals coupled to a tuning voltageinput node 13 through which a tuning voltage Vtune is inputted. Inaddition, the first and second output terminals 11 and 12 correspond toan oscillation node of a voltage-controlled oscillation node.

The tuning voltage Vtune has a voltage level ranging from about 0V to apower supply voltage (VDD). For example, the power supply voltage mayhave a voltage level of about 2.8V. Moreover, the capacitance of thefirst varactor Cv1 and the second varactor Cv2 are varied according tothe tuning voltage Vtune.

However, as shown in the graph of FIG. 2, the AMOS varactor exhibits asignificant change in the capacitance with respect to the tuning voltageVtune ranging from about −1V to about 1V. In contrast, the AMOS varactorexhibits little change in capacitance with respect to the tuning voltageVtune outside of the range of about −1V to about 1V.

When a tuning voltage Vtune ranging from about 0V to about 2.8V isinputted, the capacitance tuning range of the AMOS varactor of less than0V (for example, −1V) may not be produced. Consequently, the capacitancetuning range of the AMOS varactor may be reduced, by about one half ofthe full capacitance tuning range of the AMOS varactor, therebyrendering about half of the full capacitance tuning range of the AMOSvaractor unavailable.

Therefore, in order to use the full capacitance tuning range of the AMOSvaractor, the tuning voltage Vtune needs to range from a negativevoltage level to a positive voltage level. However, in actual use, thetuning voltage Vtune may have a voltage range between ground and a powersupply voltage (VDD).

In the conventional fine tuning circuit depicted in FIG. 3, a DC voltageoutputted from the first and second output terminals 11 and 12 isprovided to the first and second varactors Cv1 and Cv2 so that thecapacitances of the first and second varactors Cv1 and Cv2 may varyaccording to the DV voltage outputted from the first and second outputterminals 11 and 12.

Consequently, when a common noise is introduced to the DC voltageoutputted from the first and second output terminal 11 and 12, thecapacitances of the first and second varactors Cv1 and Cv2 may bemodulated so that phase noise degradation may occur due to an FMmodulation.

As described above, in the conventional fine tuning circuit, thecapacitance tuning range of the AMOS varactor may be reduced. Moreover,the conventional fine tuning circuit may suffer degradation inperformance due to, for example, phase noise degradation caused by noiseintroduced from the output terminal.

Additionally, the above difficulties may arise not only in the finetuning circuit but also in a coarse tuning circuit, which has a circuitconfiguration very similar to the fine tuning circuit.

Thus, there is a need for a frequency fine tuning circuit, and/or afrequency coarse tuning circuit for use with a voltage-controlledoscillator, in which the capacitance tuning range of a varactor isincreased while phase noise degradation due to an FM modulation isprevented.

SUMMARY OF THE INVENTION

Some example embodiments of the present invention provide a frequencyfine tuning circuit in which the capacitance tuning range of a varactoris increased while phase noise degradation due to an FM modulation isprevented.

Some example embodiments of the present invention m also provide afrequency coarse tuning circuit in which the capacitance tuning range ofa varactor is increased while phase noise degradation due to an FMmodulation is prevented.

Some example embodiments of the present invention also provide avoltage-controlled oscillator having a frequency fine tuning circuitand/or a frequency coarse tuning circuit in which the capacitance tuningrange of a varactor is increased while phase noise degradation due to anFM modulation is prevented.

In some example embodiments of the present invention, a frequency finetuning circuit for a voltage-controlled oscillator includes a firstvaractor, a second varactor and a center bias unit. The second varactoris coupled to the first varactor at a first node. The center bias unitmaintains a node voltage of the first node at a constant bias voltagelevel. The first and second varactors may include accumulation MOS(AMOS) varactors.

In further example embodiments, the frequency fine tuning circuit mayfurther include a first blocking capacitor coupled between a firstoutput terminal and the first varactor and configured to prevent noisebeing transmitted to the first varactor, and a second blocking capacitorcoupled between a second output terminal and the second varactor andconfigured to prevent noise being transmitted to the second varactor.The first and second blocking capacitors may includemetal-insulator-metal (MIM) capacitors.

In further example embodiments, a first tuning voltage inputtingterminal for providing a tuning voltage may be electrically coupled to anode where the first blocking capacitor and the first varactor arecoupled, Also, the first varactor is configured for having thecapacitance varied according to the tuning voltage. A second tuningvoltage inputting terminal for providing a tuning voltage may beelectrically coupled to the node where the second blocking capacitor andthe second varactor are coupled. Moreover, the second varactor isconfigured for having the capacitance varied according to the tuningvoltage.

In additional example embodiment, the center bias unit may include abias voltage generating unit coupled to the first node and configured todetermine a voltage level of the first node, as well as a switch unitconfigured to supply a power supply voltage to the bias voltagegenerating unit in response to a voltage-controlled oscillation enablesignal that is externally provided.

Furthermore, the bias voltage generating unit may include a diode and aresistor that is serially coupled to each other. The bias voltagegenerating unit may include at least one diode serially coupled.Alternatively, the bias voltage generating unit may include a resistor.

In further example embodiments, the capacitance of the first varactorand the capacitance of the second varactor may be varied according tothe tuning voltage ranging from a ground voltage level to a power supplyvoltage level. The voltage level at the first node may be maintained atone half of a power supply voltage.

In some example embodiments of the present invention, a frequency coarsetuning circuit for use in a voltage-controlled oscillator may include afirst varactor, a first blocking capacitor, a second varactor, a secondblocking capacitor and a center bias unit. The first varactor isconfigured to have the capacitance varied according to a bit valueexternally inputted. The first blocking capacitor is coupled between afirst output terminal and the first varactor and configured to preventnoise from being transmitted to the first varactor. The second varactoris configured to have the capacitance varied according to the bit valueexternally inputted and the second varactor is coupled to the firstvaractor at a first node. The second blocking capacitor is coupledbetween a second output terminal and the second varactor and configuredto prevent noise from being transmitted to the second varactor. Thecenter bias unit maintains a node voltage of the first node at aconstant bias voltage level.

In further example embodiments, the first and second varactors includeaccumulation MOS (AMOS) varactors, and the first and second blockingcapacitors may include metal-insulator-metal (MIM) capacitors.

The first data input terminal may be coupled to a node where the firstvaractor and the first blocking capacitor are coupled, and the bit valuemay be inputted through the first data input terminal. A second datainput terminal may be coupled to a node where the second varactor andthe second blocking capacitor are coupled, and the bit value may beinputted through the second data input terminal. The first and thesecond data input terminals may be coupled to an inverter so that aninverted bit value is outputted.

In further example embodiments, a logic low level of the bit value maybe a ground voltage level, and a logic high level of the bit value maybe a power supply voltage level. The voltage level at the first node maybe maintained at one half of a power supply voltage. Therefore, thefirst and second varactors may be controlled by the one half of a powersupply voltage.

In some example embodiments of the present invention, avoltage-controlled oscillator may include an active circuit, an inductorand a fine tuning circuit. The active circuit has a first oscillationnode and a second oscillation node. The inductor is coupled between thefirst and second oscillation nodes. The fine tuning circuit is coupledbetween the first and second oscillation nodes. The fine tuning circuitincludes a first varactor configured to have the capacitance variedaccording to a tuning voltage externally provided, a second varactorcoupled to the first varactor at a first node where the second varactoris configured to have the capacitance varied according to the tuningvoltage, and a center bias unit configured to maintain a node voltage ofthe first node at a constant bias voltage level.

In further example embodiments, the fine tuning circuit may furtherinclude a first blocking capacitor coupled between the first oscillationnode and the first varactor and configured to prevent noise beingtransmitted to the first varactor, and a second blocking capacitorcoupled between the second oscillation node and the second varactor andconfigured to prevent noise being transmitted to the second varactor.

In additional example embodiments, a voltage-controlled oscillator mayinclude an active circuit, an inductor and at least one coarse tuningcircuit. The active circuit may have a first oscillation node and asecond oscillation node. The inductor may be coupled between the firstand second oscillation nodes. The at least one coarse tuning circuit maybe coupled between the first and second oscillation nodes. Moreover, theat least one coarse tuning circuit includes a first varactor, a secondvaractor and a center bias unit. Further, the first varactor may beconfigured to have the capacitance varied according to a bit valueexternally provided. The second varactor may be configured to have thecapacitance varied according to the bit value, and may be coupled to thefirst varactor at a first node. The center bias unit may maintain a nodevoltage of the first node at a constant bias voltage level.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more apparent to those of ordinaryskill in the art by describing, in detail, example embodiments thereofwith reference to the attached drawings, wherein like elements arerepresented by like reference numerals, which are given by way ofillustration only and thus do not limit the example embodiments of thepresent invention.

FIG. 1 is a cross-sectional view illustrating a typical structure of aconventional accumulation MOS (AMOS) varactor.

FIG. 2 is a graph illustrating a tuning characteristic of theconventional AMOS varactor in FIG. 1.

FIG. 3 is a circuit diagram illustrating a conventional fine tuningcircuit.

FIG. 4 is a circuit diagram illustrating a fine tuning circuit of avoltage-controlled oscillator according to an example embodiment of thepresent invention.

FIG. 5 is a circuit diagram illustrating a fine tuning circuit accordingto an example embodiment of the present invention.

FIG. 6 is a circuit diagram illustrating a fine tuning circuit accordingto an example embodiment of the present invention.

FIG. 7 is a schematic view illustrating a coarse tuning unit of avoltage-controlled oscillator.

FIG. 8 is a circuit diagram illustrating the coarse tuning circuit inFIG. 7 according to an example embodiment of the present invention.

FIG. 9 is a circuit diagram illustrating the coarse tuning circuit inFIG. 7 according to an example embodiment of the present invention.

FIG. 10 is a circuit diagram illustrating the coarse tuning circuit inFIG. 7 according to an example embodiment of the present invention.

FIG. 11 is a circuit diagram illustrating a voltage-controlledoscillator having the fine tuning circuit as shown in FIG. 4, and thecoarse tuning circuit as shown in FIG. 7, according to an exampleembodiment of the present invention.

DESCRIPTION OF THE EXAMPLE EMBODIMENTS OF THE INVENTION

Hereinafter, the example present invention will be explained in detailwith reference to the accompanying drawings.

FIG. 4 is a circuit diagram illustrating a structure of a fine tuningcircuit 100 of a voltage-controlled oscillator according to an exampleembodiment of the present invention.

Herein, a gate terminal of the varactor may be referred to as a firstterminal and source. Also, a drain terminal may be referred herein to asa second terminal. The above definitions may similarly be applied withregard to a blocking capacitor.

Referring to FIG. 4, the fine tuning circuit 100 according to an exampleembodiment of the present invention may include a first blockingcapacitor C21, a first varactor Cv21, a second blocking capacitor C22, asecond varactor Cv22 and a center bias unit 110.

The first and second varactors Cv21 and Cv22 may include an accumulationMOS (AMOS) varactor and the first and second blocking capacitors C21 andC22 may include a metal-insulator-metal (MIM) capacitor.

The first varactor Cv21 is serially connected to the first blockingcapacitor C21. The first varactor Cv21 has a first terminal electricallycoupled to a second terminal of the first blocking capacitor C21 and asecond terminal electrically coupled to a second terminal of the secondvaractor Cv22.

A node between the first varactor Cv21 and the first blocking capacitorC21 is electrically connected to a first tuning voltage inputtingterminal 8 through which a tuning voltage Vtune for adjusting thecapacitance of the first varactor Cv21 is provided. A resistor may becoupled to the first tuning voltage inputting terminal 8. Therefore, thecapacitance of the first varactor Cv21 may be varied according to thetuning voltage Vtune inputted from the first tuning voltage inputtingterminal 8.

The first blocking capacitor C21 has a second terminal electricallycoupled to the first terminal of the first varactor Cv21 and a firstterminal coupled to a first output terminal 1. Herein, the firstblocking capacitor C21 may function to block a DC component outputtedfrom the first output terminal 1 so that the first varactor Cv21 may notreceive any noise.

The second varactor Cv22 is serially coupled to the second blockingcapacitor C22. Herein, the second varactor Cv22 has a first terminalelectrically coupled to a second terminal of the second blockingcapacitor C22 and a second terminal electrically coupled to the secondterminal of the first varactor Cv21.

A node between the second varactor Cv22 and the second blockingcapacitor C22 is electrically connected to a second tuning voltageinputting terminal 9 through which the tuning voltage Vtune foradjusting the capacitance of the second varactor Cv22 is provided. Aresistor Rv may be coupled between the second tuning voltage inputtingterminal 9 and a node where the second varactor Cv22 and the secondblocking capacitor C22 are coupled. Therefore, the capacitance of thesecond varactor Cv22 may be varied according to the tuning voltage Vtuneinputted from the second tuning voltage inputting terminal 9.

The second blocking capacitor C22 has a second terminal electricallycoupled to the first terminal of the second varactor Cv22 and a firstterminal coupled to a second output terminal 2. Herein, the secondblocking capacitor C22 may function to block a DC component outputtedfrom the second output terminal 2 so that the second varactor Cv22 maynot receive any noise.

Thus, the first blocking capacitor C21 and the first varactor Cv21 forma symmetric structure and the second blocking capacitor C22 and thesecond varactor Cv22 form a symmetric structure.

A bias node A is formed between the first and second varactors Cv21 andCv22. A voltage level of the bias node A is maintained constant by acenter bias unit 110.

The center bias unit 110 may include a MOS transistor Ma and a biasvoltage generating unit 120 for determining a voltage level of the biasnode A. The MOS transistor Ma may have a drain terminal electricallycoupled to a power supply voltage VDD, a source terminal electricallycoupled to the bias node A and a gate terminal for receiving avoltage-controlled oscillator enable signal EN. The bias voltagegenerating unit 120 is coupled to the bias node A. In one exampleembodiment, a resistor R1 may be coupled to the drain terminal of theMOS transistor Ma.

The bias voltage generating unit 120 includes a diode Da, a resistor Rathat is serially coupled to the diode Da, and a bypass capacitor C23that is parallel coupled to the diode Da and the resistor Ra. The bypasscapacitor C23 is used to bypass an AC component.

The operation of the fine tuning circuit 100 in FIG. 4 will bedescribed.

When the voltage-controlled oscillator enable signal EN is turned on toperform the fine tuning, the MOS transistor Ma of the center bias unit110 is turned on so that a current I flows to the bias voltagegenerating unit 120.

As the bias voltage generating unit 120 includes the diode Da and theresistor Ra, the voltage level of the bias node A may be determined by athreshold voltage VDath of the diode Da, the current I and theresistance of the resistor Ra and thus, represented by the followingexpression 1.VA=VDath+I×Ra  [Expression 1]

For example, the threshold voltage VDath of the diode Da and theresistance of the resistor Ra may be determined such that a voltagelevel Va of the bias node A becomes +VDD/2. For example, when the VDD is2.8V, the threshold voltage VDath of the diode Da and the resistance ofthe resistor Ra may have a value such that the voltage level Va of thebias node A is +1.4V.

When the tuning voltage Vtune is applied to the first tuning voltageinputting terminal 8 between the first varactor Cv21 and the firstblocking capacitor C21, the capacitance of the first varactor Cv21 maybe varied according to the tuning voltage Vtune. Similarly, when thetuning voltage Vtune is applied to the second tuning voltage inputtingterminal 9 between the second varactor Cv22 and the second blockingcapacitor C22, the capacitance of the second varactor Cv22 may be variedaccording to the tuning voltage Vtune.

The tuning voltage Vtune may have a voltage level ranging between aground voltage and the power supply voltage VDD. For example, the powersupply voltage VDD may have a voltage level of about 2.8V.

Therefore, when the tuning voltage Vtune having a voltage level betweenabout 0V and about 2.8V is applied to the first and second varactorsCv21 and Cv22 and the voltage level of the bias node Va has a voltagelevel of 1.4V, the capacitances of the first and second varactors Cv21and Cv22 may be varied according to the voltage difference between thetuning voltage Vtune and the voltage at the bias node A, i.e., −1.4V(0V-1.4V) to +1.4V (2.8V-1.4V). Therefore, according to an exampleembodiment of the present invention, nearly all the capacitance tuningrange of the AMOS varactor illustrated in the tuning characteristicgraph of FIG. 2 may be available.

Furthermore, by using the first and second blocking capacitors C21 andC22, the DC component outputted from the first and second outputterminals 1 and 2 may not be transmitted to the first and secondvaractors Cv21 and Cv22. Consequently, the performance of the varactorsCv21 and Cv22 may be improved and the fine tuning may be performedstably.

The bias voltage generating unit 120 may be implemented using the diodeDa and the resistor Ra as described in FIG. 4. Alternatively, the biasvoltage generating unit 120 may be implemented using a diode only or aresistor only.

FIG. 5 is a circuit diagram illustrating a fine tuning circuit 200according to another example embodiment of the present invention.

Referring to FIG. 5, a bias voltage generating unit 220 of the centerbias unit 210 includes a first diode D1 and a second diode D2. Thevoltage level of the bias node A is determined by a summation of athreshold voltage VDth1 of the first diode D1 and a threshold voltageVDth2 of the second diode D2 as represented by the following expression2.VA=VDth1+VDth2  [Expression 2]

FIG. 6 is a circuit diagram illustrating a fine tuning circuit 300according to an example embodiment of the present invention.

Referring to FIG. 6, a bias voltage generating unit 320 of the centerbias unit 310 includes a single resistor R. In this example embodiment,the voltage level VA of the bias node A may be determined by the currentI flowing through the resistor R of the bias voltage generating unit 320and the resistance of the resistor R, which is represented by thefollowing expression 3.VA=I×R  [Expression 3]

As described above, using the fine tuning devices 100, 200 and 300, theAMOS varactors Cv21 and Cv22 may have a full capacitance tuning rangewhile the DC noise from the output terminals 1 and 2 are removed.

Further, a coarse tuning circuit that may adjust the oscillation of afrequency based on an input digital bit value is provided according toan example embodiment of the present invention. The coarse tuningcircuit according to an example embodiment of the present invention willbe described with reference to FIG. 7.

Referring to FIG. 7, the coarse tuning unit 400 includes N+1 coarsetuning circuits 500, each of which corresponds to each bit of inputtedN-bit digital data.

Each of the coarse tuning circuits 500 has a first terminal coupled to afirst output terminal 3 and a second terminal coupled to a second outputterminal 4. Each of the coarse tuning circuits 500 includes a varactorthat is binary weighted according to a corresponding bit place ofreceived digital data. For example, when the varactor of the coarsetuning circuit 500 that receives a data bit of a first bit place has acapacitance of 2⁰×C, the varactor of the coarse tuning circuit 500 thatreceives a data bit of an Nth place may have a capacitance of 2^(N−1)×C.

FIG. 8 is a circuit diagram illustrating the coarse tuning circuit ofFIG. 7 according to an example embodiment of the present invention.

Referring to FIG. 8, the coarse tuning circuit 500 according to anexample embodiment of the present invention includes a first blockingcapacitor C31, a first varactor Cv31, a second blocking capacitor C32, asecond varactor Cv32 and a center bias unit 510. For example, the firstand second varactors Cv31 and Cv32 may include an AMOS varactor and thefirst and second blocking capacitors C31 and C32 may include an MIMcapacitor.

The first varactor Cv31 is serially coupled to the first blockingcapacitor C31. For instance, in this example embodiment, the firstvaractor Cv31 has a first terminal coupled to a second terminal of thefirst blocking capacitor C31 and a second terminal coupled to a secondterminal of the second varactor Cv32.

A data input terminal 530 is electrically connected to a node where thefirst varactor Cv31 and the first blocking capacitor C31 are coupled. Aninput bit data Bn is inputted to the data input terminal 530 to adjustthe capacitance of the first varactor Cv31.

The first blocking capacitor C31 has a second terminal electricallycoupled to a first terminal of the first varactor Cv31 and a firstterminal coupled to the first output terminal 3. Herein, the firstblocking capacitor C31 may function to block a DC component outputtedfrom the first output terminal 3 so that the first varactor Cv31 may notreceive any noise.

The second varactor Cv32 is serially coupled to the second blockingcapacitor C32. For example, in this example embodiment, the secondvaractor Cv32 has a first terminal coupled to a second terminal of thesecond blocking capacitor C32 and a second terminal coupled to a secondterminal of the first varactor Cv31.

A data input terminal 530 is electrically connected to a node where thesecond varactor Cv32 and the second blocking capacitor C32 are coupled.An input bit data Bn is inputted to the data input terminal 530 toadjust the capacitance of the second varactor Cv32.

An inverter may be used to invert the input data bit Bn inputted throughthe data input terminal 530. Therefore, an inverted bit value Bn may beprovided to the first and second varactors Cv31 and Cv32.

The second blocking capacitor C32 has a second terminal electricallycoupled to a first terminal of the second varactor Cv32 and a firstterminal coupled to the second output terminal 4. Herein, the secondblocking capacitor C32 may function to block a DC component outputtedfrom the second output terminal 4 so that the second varactor Cv32 maynot receive any noise.

As described above, the first blocking capacitor C31 and the firstvaractor Cv31 form a symmetric structure and the second blockingcapacitor C32 and the second varactor Cv32 form a symmetric structure.

A bias node A is formed between the first and second varactors Cv31 andCv32. A voltage level of the bias node A is maintained constant by thecenter bias unit 510.

The center bias unit 510 may include a bias voltage generating unit 520for determining a voltage level of the bias node A and a control switch511 for controlling the supply of a power supply voltage VDD to the biasvoltage generating unit 520. The control switch 511 may be turned on oroff by a voltage-controlled oscillation enable signal.

The bias voltage generating unit 520 may include a diode Da, a resistorRa that is serially coupled to the diode Da, and a bypass capacitor C33that is parallel coupled to the diode Da and the resistor Ra. The bypasscapacitor C33 is used to bypass an AC component.

The operation of the fine tuning circuit 500 in FIG. 8 will be describedbelow.

When the control switch 511 of the center bias unit 510 is turned on,the power supply voltage VDD is provided to the bias voltage generatingunit 520 so that a current I flows therethrough. As the bias voltagegenerating unit 520 includes the diode Da and the resistor Ra, thevoltage at the bias node A may be determined by a threshold voltageVDath of the diode Da, the current I and the resistance of the resistorRa, which is given as the above expression 1.

For example, the threshold voltage VDath of the diode Da and theresistance of the resistor Ra may be determined such that a voltagelevel Va of the bias node A becomes +VDD/2. For example, when the VDD is2.8V, the threshold voltage VDath of the diode Da and the resistance ofthe resistor Ra may have a value such that the voltage level Va of thebias node A is +1.4V.

When the input bit value Bn having a logic high level voltage isinputted to the data input terminal 530, the input bit value Bn isinverted so that a logic low level signal is provided to the first andsecond varactors Cv31 and Cv32.

For example, a logic low level signal may have a voltage level of abouta ground voltage, i.e., 0V and a logic high level signal may have avoltage level of about a power supply voltage VDD, e.g., 2.8V.

When the low level signal, e.g., 0V, is inputted to the first and secondvaractors Cv31 and Cv32, the capacitances of the first and secondvaractors Cv31 and Cv32 are determined by a voltage level of −VDD/2,e.g., about −1.4V (0V-1.4V), because the voltage level Va at the biasnode A is maintained at about +1.4V.

When the input bit value Bn having a logic low level voltage is inputtedto the data input terminal 530, the input bit value Bn is inverted sothat a logic high level signal is provided to the first and secondvaractors Cv31 and Cv32.

When the high level signal, e.g., about 2.8V, is inputted to the firstand second varactors Cv31 and Cv32, the capacitances of the first andsecond varactors Cv31 and Cv32 are determined by a voltage level of+VDD/2, e.g., about +1.4V (2.8V-1.4V), because the voltage level Va atthe bias node A is maintained at about +1.4V.

In addition, by using the first and second blocking capacitors C31 andC32, the DC component outputted from the first and second outputterminals 3 and 4 may not be transmitted to the first and secondvaractors Cv31 and Cv32.

Moreover, the bias voltage generating unit 520 may be implemented usingthe diode Da and the resistor Ra as described in FIG. 8. Alternatively,the bias voltage generating unit 520 may be implemented using a diodeonly or a resistor only.

FIG. 9 is a circuit diagram illustrating the coarse tuning circuit inFIG. 7 according to another example embodiment of the present invention.

Referring to FIG. 9, in a coarse tuning circuit 600, a bias voltagegenerating unit 620 of the center bias unit 610 includes a first diodeD1 and a second diode D2. The bias voltage generating unit 620 mayfurther include a bypass capacitor C33 that is parallel coupled to thefirst and second diodes D1 and D2 to bypass an AC component. The voltagelevel Va at the bias node A is determined by a sum of the thresholdvoltage VDth1 of the first diode D1 and the threshold voltage VDth2 ofthe second diode D2, which is given as the above expression 2.

FIG. 10 is a circuit diagram illustrating the coarse tuning circuit inFIG. 7 according to another example embodiment of the present invention.

Referring to FIG. 10, in a course tuning circuit 700, a bias voltagegenerating unit 720 of the center bias unit 710 includes a singleresistor R. The bias voltage generating unit 720 may further include abypass capacitor C33 that is parallel coupled to the resistor R tobypass an AC component. In this case, the voltage level Va at the biasnode A is determined by the current I flowing through the resistor R andthe resistance of the resistor R, which is given as the above expression3.

The fine tuning circuits 100, 200 and 300 in FIGS. 4 through 6 and thecoarse tuning circuits 500, 600 and 700 in FIGS. 8 through 10 may beeasily applied to a voltage-controlled oscillator.

FIG. 11 is a circuit diagram illustrating a voltage-controlledoscillator 1000 having the fine tuning circuit 100 in FIG. 4 and thecoarse tuning unit 400 in FIG. 7 according to an example embodiment ofthe present invention.

Referring to FIG. 11, in the voltage-controlled oscillator 1000, firstand second output terminals 1 and 2 of the fine tuning circuit 100 arecoupled to first and second output terminals 3 and 4 of the coarsetuning unit 400, respectively, and the first and second output terminals3 and 4 of the coarse tuning unit 400 are coupled to first and secondoscillation nodes 910 and 920 of an active circuit 900, respectively. Aninductor L3 may be disposed between the first and second oscillationnodes 910 and 920 so that a frequency oscillation may occur through anLC resonance.

The active circuit 900 may include a pair of cross-coupled transistorsM3 and M4 to generate a negative transconductor for frequencyoscillation. The voltage-controlled oscillator 100 may further include abias circuit 930 comprising MOS transistors M5 and M6, an inductor L2, acapacitor C2, and a low pass filter 940 comprising a resistor R1 and acapacitor C1.

As described above, according to the example embodiments of the presentinvention, a bias node where two varactors of the fine tuning circuitare coupled may be maintained at a particular voltage level, forexample, +VDD/2, so that the fine tuning circuit may have an increasedcapacitance tuning range of the AMOS varactor. In addition, by using ablocking capacitor disposed between an output terminal and the varactor,an unnecessary DC component may be removed so that the frequency tuningis stably performed and the performance of a voltage-controlledoscillator is improved.

Having described the example embodiments of the present invention, it isfurther noted that it is readily apparent to those of reasonable skillin the art that various modifications may be made without departing fromthe spirit and scope of the invention which is defined by the metes andbounds of the appended claims.

1. A frequency fine tuning circuit for use in a voltage-controlledoscillator, comprising: a first varactor; a second varactor coupled tothe first varactor at a first node; and a center bias unit configured tomaintain a node voltage of the first node at a constant bias voltagelevel , the constant bias voltage level substantially corresponding to amean value of a range of a tuning voltage, wherein the center bias unitcomprises: a bias voltage generating unit coupled to the first node, thebias voltage generating unit configured to determine a voltage level ofthe first node; and a switch unit configured to supply a power supplyvoltage to the bias voltage generating unit in response to avoltage-controlled oscillation enable signal that is externallyprovided.
 2. The frequency fine tuning circuit of claim 1, wherein thefirst and second varactors comprise accumulation MOS (AMOS) varactors.3. The frequency fine tuning circuit of claim 1, further comprising: afirst blocking capacitor coupled between a first output terminal and thefirst varactor, the first blocking capacitor configured to prevent noisefrom being transmitted to the first varactor; and a second blockingcapacitor coupled between a second output terminal and the secondvaractor, the second blocking capacitor configured to prevent noise frombeing transmitted to the second varactor.
 4. The frequency tuningcircuit of claim 3, wherein the first and second blocking capacitorscomprise insulator metal (MIM) capacitors.
 5. The frequency fine tuningcircuit of claim 3, wherein the first varactor has a first terminalelectrically coupled to a second terminal of the first blockingcapacitor, and a second terminal electrically coupled to a secondterminal of the second varactor.
 6. The frequency fine tuning circuit ofclaim 3, wherein a first tuning voltage inputting terminal for providingthe tuning voltage is electrically coupled to a node where the firstblocking capacitor and the first varactor are coupled, and wherein thefirst varactor is configured to have a capacitance varied according tothe tuning voltage.
 7. The frequency fine tuning circuit of claim 3,wherein the second varactor has a first terminal electrically coupled toa second terminal of the second blocking capacitor, and a secondterminal electrically coupled to a second terminal of the firstvaractor.
 8. The frequency fine tuning circuit of claim 3, wherein asecond tuning voltage inputting terminal for providing the tuningvoltage is electrically coupled to a node where the second blockingcapacitor and the second varactor are coupled, and wherein a capacitanceof the second varactor is varied according to the tuning voltage.
 9. Thefrequency fine tuning circuit of claim 1, wherein the switch unitincludes a MOS transistor, the MOS transistor having a first terminalcoupled to the power supply voltage, a second terminal coupled to thefirst node, and a control terminal for receiving the voltage-controlledoscillation enable signal EN.
 10. The frequency fine tuning circuit ofclaim 1, wherein the bias voltage generating unit includes a diode and aresistor that are serially coupled to each other.
 11. The frequency finetuning circuit of claim 1, wherein the bias voltage generating unitcomprises at least one diode serially coupled.
 12. The frequency finetuning circuit of claim 1, wherein the bias voltage generating unitcomprises a resistor.
 13. The frequency fine tuning circuit of claim 1,wherein the bias voltage generating unit further comprises a bypasscapacitor configured to bypass an AC component.
 14. A frequency coarsetuning circuit for use in a voltage-controlled oscillator, comprising: afirst varactor configured to have a capacitance varied according to abit value that is externally inputted; a first blocking capacitorcoupled between a first output terminal and the first varactor, thefirst blocking capacitor configured to prevent noise from beingtransmitted to the first varactor; a second varactor configured to havea capacitance varied according to the bit value that is externallyinputted, the second varactor being coupled to the first varactor at afirst node; a second blocking capacitor coupled between a second outputterminal and the second varactor, the second blocking capacitorconfigured to prevent noise from being transmitted to the secondvaractor; and a center bias unit configured to maintain a node voltageof the first node at a constant bias voltage level ,the constant biasvoltage level substantially corresponding to a mean value of a range ofa tuning voltage, wherein the center bias unit comprises: a bias voltagegenerating unit coupled to the first node, the bias voltage generatingunit configured to determine a voltage level of the first node; and aswitch unit configured to supply a power supply voltage to the biasvoltage generating unit in response to a voltage-controlled oscillationenable signal that is externally provided.
 15. The frequency coarsetuning circuit of claim 14, wherein the first and second varactorscomprise accumulation MOS (AMOS) varactors.
 16. The frequency coarsetuning circuit of claim 14, wherein the first and second blockingcapacitors comprise metal-insulator-metal (MIM) capacitors.
 17. Thefrequency coarse tuning circuit of claim 14, wherein the first varactorhas a first terminal electrically coupled to a second terminal of thefirst blocking capacitor, and a second terminal electrically coupled toa second terminal of the second varactor.
 18. The frequency coarsetuning circuit of claim 14, wherein a first data input terminal iscoupled to a node where the first varactor and the first blockingcapacitor are coupled, and wherein the first data input terminal isconfigured to receive the bit value when inputted therethrough.
 19. Thefrequency coarse tuning circuit of claim 18, wherein the first datainput terminal is coupled to an inverter so that an inverted bit valueis outputted.
 20. The frequency coarse tuning circuit of claim 14,wherein the second varactor has a first terminal electrically coupled toa second terminal of the second blocking capacitor and a second terminalelectrically coupled to a second terminal of the first varactor.
 21. Thefrequency coarse tuning circuit of claim 14, wherein a second data inputterminal is coupled to a node where the second varactor and the secondblocking capacitor are coupled, and wherein the second data inputterminal is configured to receive the bit value when inputtedtherethrough.
 22. The frequency coarse tuning circuit of claim 21,wherein the second data input terminal is coupled to an inverter so thatan inverted bit value is outputted.
 23. The frequency coarse tuningcircuit of claim 14, wherein the bias voltage generating unit comprisesa diode and a resistor that are serially coupled to each other.
 24. Thefrequency coarse tuning circuit of claim 14, wherein the bias voltagegenerating unit comprises at least one diode serially coupled.
 25. Thefrequency coarse tuning circuit of claim 14, wherein the bias voltagegenerating unit comprises a resistor.
 26. The frequency fine tuningcircuit of claim 14, wherein the bias voltage generating unit furthercomprises a bypass capacitor configured to bypass an AC component. 27.The frequency coarse tuning circuit of claim 14, wherein a logic lowlevel of the bit value corresponds to a ground voltage level and a logichigh level of the bit value corresponds to a power supply voltage level.28. A voltage-controlled oscillator comprising: an active circuit havinga first oscillation node and a second oscillation node; an inductorcoupled between the first and second oscillation nodes; and a finetuning circuit coupled between the first and second oscillation nodes,wherein the fine tuning circuit comprises: a first varactor configuredto have a capacitance varied according to a tuning voltage that isexternally provided; a second varactor configured to have a capacitancevaried according to the tuning voltage, the second varactor beingcoupled to the first varactor at a first node; and a center bias unitconfigured to maintain a node voltage of the first node at a constantbias voltage level, the constant bias voltage level substantiallycorresponding to a mean value of a range of the tuning voltage, whereinthe center bias unit comprises: a bias voltage generating unit coupledto the first node, the bias voltage generating unit configured todetermine a voltage level of the first node; and a switch unitconfigured to supply a power supply voltage to the bias voltagegenerating unit in response to a voltage-controlled oscillation enablesignal that is externally provided.
 29. The voltage-controlledoscillator of claim 28, wherein the fine tuning circuit furthercomprises: a first blocking capacitor coupled between the firstoscillation node and the first varactor, the first blocking capacitorconfigured to prevent noise from being transmitted to the firstvaractor; and a second blocking capacitor coupled between the secondoscillation node and the second varactor, the second blocking capacitorconfigured to prevent noise from being transmitted to the secondvaractor.
 30. The voltage-controlled oscillator of claim 28, wherein thetuning voltage ranges from a ground voltage level to a power supplyvoltage level.
 31. A voltage-controlled oscillator comprising: an activecircuit having a first oscillation node and a second oscillation node;an inductor coupled between the first and second oscillation nodes; andat least one coarse tuning circuit coupled between the first and secondoscillation nodes, wherein the at least one coarse tuning circuitcomprises: a first varactor configured to have a capacitance variedaccording to a bit value externally provided; a second varactorconfigured to have a capacitance varied according to the bit value, thesecond varactor being coupled to the first varactor at a first node; anda center bias unit configured to maintain a node voltage of the firstnode at a constant bias voltage level, the constant bias voltage levelsubstantially corresponding to a mean value of a range of the tuningvoltage, wherein the center bias unit comprises: a bias voltagegenerating unit coupled to the first node, the bias voltage generatingunit configured to determine a voltage level of the first node; and aswitch unit configured to supply a power supply voltage to the biasvoltage generating unit in response to a voltage-controlled oscillationenable signal that is externally provided.
 32. The voltage-controlledoscillator of claim 31, wherein each of the at least one coarse tuningcircuit further comprises: a first blocking capacitor coupled betweenthe first oscillation node and the first varactor, the first blockingcapacitor configured to prevent noise from being transmitted to thefirst varactor; and a second blocking capacitor coupled between thesecond oscillation node and the second varactor, the second blockingcapacitor configured to prevent noise from being transmitted to thesecond varactor.
 33. The voltage-controlled oscillator of claim 31,wherein a logic low level of the bit value corresponds to a groundvoltage level and a logic high level of the bit value corresponds to apower supply voltage level.